A STOCHASTIC-MODEL OF LOW FLOWS

A model is developed for annual low flow hydrographs. Its two primary components reflect the fact that hydrologic processes during streamflo w rise (function of water input) and recession (function of basin stor age) are different. Durations of periods of rise (wet intervals) and r ecession (dry intervals) are modelled by discrete probability distribu tions negative binomial for dry intervals and negative binomial or mod ified logarithmic series for wet intervals depending on goodness of fi t. During wet intervals, the total inflow is modelled by the lognormal distribution and daily amounts are allocated according to a pattern-a veraged model. During dry intervals, the flow recedes according to a d eterministic-stochastic recession model. The model was applied to thre e Canadian basins with drainage area ranging from 2210 to 22000 km(2) to generate 50 realizations of low flow hydrographs. The resulting two standard-error confidence band for the simulated probability distribu tion of annual minimum 7-day flows enclosed the probability distributi on estimated from the observed record. A sensitivity analysis for the three basins revealed that in addition to the recession submodel, the most important submodel is that describing seasonality. The state of t he basin at the beginning of the low flow period is of marginal import ance and the daily distribution of input is unimportant.